Impact on water resources

These stores of water are increasing in some regions and decreasing in others, for various reasons. Such phenomena can be linked to changes in temperature (acceleration of glacial retreat, thawing of permafrost) and to changing precipitation caused by global climate change. Their impact on water resources depends on variations in volume or surface fl ow and on changes in river basin management systems, and on measures taken to adapt to climate change. Regional differences and dissimilar adaptive abilities affect their vulnerability. It is therefore necessary to quantify their respective effects very precisely. Conversely, changes in hydrological cycles affect the economy, society and the environment. The 'good health' of many ecosystems is very sensitive to the quantity and quality of the water in rivers, lakes and aquifers. Their global distribution and their variability over space and time are still poorly understood due to a lack of in situ data on the scale of the planet. This is why space missions have been used more systematically over the last twenty years.

The rise in the level of the oceans threatens the coastal metropolises. Here, Mumbai (former Bombay), capital of Maharashtra state (India) with an estimated population of more than15 million inhabitants. Image taken by the instrument LISS III on board the Indian satellite IRS-P6 in 2004. Courtesy National Remote Sensing Centre, ISRO, Government of India

How are hydrological cycles affected by climate change? Water budgets are very sensitive to the slightest changes, on daily to decadal scales. Such variability is chiefl y driven by precipitation. Not only change from one year to the next but also very sudden change over a few days plays a very important role in flooding, for instance. Such change also has an impact on the frequency and duration of periods of drought that can affect a region. Changes in precipitation are themselves very dependent on variations in the transfer of water into the atmosphere, which is partly linked to variations in ocean temperatures. Thanks to their climate models, scientists can explore every last aspect of this mechanism with the aim of better forecasting future change.

In addition, lower precipitation will lead to a greater risk of drought, which can bring about a fall in soil moisture, stored groundwater and river flow, and even cause lakes to dry up. Not forgetting the effects it has on human populations. Variations in temperature are also a factor of hydrological variability, since they alter potential precipitation: higher temperatures increase evapotranspiration by increasing the atmosphere's ability to hold water vapour, with contrasting effects from one region to another. Temperature is also a key variable for the study of soil moisture, which depends at a given moment on the evaporation rate, on recharge and discharge in deeper aquifers, and on surface runoff.

Refining long-term trends
In the absence of any dedicated space missions, soil moisture on the scale of the Earth is only known via climate models. However, it is also important to emphasize the lack of current and historical global data, and the fact that the models are based on a certain number of hypotheses and climate variables (wind, sunshine and precipitation). This leads to greater uncertainty and to a limitation in the estimation of long-term trends. It should prove possible to refi ne models for water transfer between the continents and atmosphere thanks to the European space mission SMOS, whose goal is to map water held in soils and its variability over time. The increase in the temperature of the atmosphere will modify runoff linked to the melting of continental glaciers, alter snowfall episodes, increase thawing of permafrost and increase surface evaporation from lakes. For example, in Siberia the period of snowfall has shortened by several weeks, lakes are thawing earlier, and permafrost is melting on a large scale. Surface snow is also melting earlier, triggering floods earlier in the season. In addition, satellite images have revealed a decrease in the area of over ten thousand 'small' lakes in Siberia.